This invention relates to linear saturated polyester compositions. More particularly, the invention is directed to a linear saturated polyester composition containing at least one ester of a high molecular weight polyfunctional alcohol.
Molding formulations based on linear saturated polyesters, such as polyethylene terephthalate, should result in a molded product having good physical properties including flexural strength, modulus, tensile strength, and impact properties. The molding compound should have good molding properties, including a melt flow index for sufficient flow into the mold, good mold release properties and good surface finish appearance. The molded article should be crystalline and warp resistant.
It is desirable that satisfactory properties be attained using water heated molds. That is, molds heated to temperatures between 76.7.degree. C. (170.degree. F.) to about 110.degree. C. (230.degree. F.). In order to accomplish this, it is desirable for crystallization to begin at as high a temperature as possible upon the cooling of the molten polyester which was fed into the mold, and continue during the cooling to as low a temperature as possible. T.sub.cc is a measurement to determine at what temperature crystals first appear upon cooling from the melt. T.sub.ch is a measurement which indicates the temperature at which crystallization is no longer occurring upon cooling. It has been found that mold appearance and mold release properties can be related to T.sub.ch. T.sub.ch is determined by measuring the temperature at which crystals appear upon heating an amorphous piece of polyester. T.sub.cc and and T.sub.ch can be measured using a Differential Scanning Calorimeter.
A variety of additives are disclosed in the art for use with linear saturated polyester compositions. Two important classes of additives include nucleators and plasticizers. Plasticizers include a variety of esters such as those disclosed in U.S. Pat. Nos. 4,223,125 and 4,435,546. These patents describe the use of esters of alcohols having up to 20 carbon atoms and preferably having a carbon bond to ester bond ratio of between 4 and 15, inclusive of the carbonyl atom.
It is known to use nucleating agents in crystallizable polymers, such as linear saturated polyesters of aromatic dicarboxylic acids. U.S. Pat. Nos. 3,435,093; 3,516,957; and 3,639,527 disclose various approaches to molding thermoplastic compositions of linear saturated polyesters of aromatic dicarboxylic acids, and are particularly applicable to polyethylene terephthalate. These patents generally disclose the use of salts of hydrocarbon and polymeric carboxylic acids as nucleating agents for linear saturated polyesters. Great Britain Patent No. 1,315,699 discloses the use of low molecular weight sodium, lithium or barium salts of mono- or polycarboxylic acids used with solid, inert inorganic substances.
The use of organic esters in combination with nucleators is disclosed in U.S. Pat. Nos. 3,516,957; 4,352,904; 4,486,564; 4,429,067; and 4,548,978. These patents disclose the use of a variety of plasticizers including specific ester compounds used in combination with other materials.